1. Field of the Invention
The present invention relates to power feeding circuits, and more particularly, to a switched mode power supply device adapted for low current drain.
2. Description of the Related Art
Portable electronic circuits develop and spread. They are powered by batteries, which supply them with the energy necessary to their operation. The designing of these batteries is also constantly developing.
The field of mobile telephony particularly illustrates the development of mobile electronics powered by electric batteries. In spite of their miniaturization, cellular phones meeting the Global Mobile Service standard require an important ringing current in operating mode. On the contrary, in suspend mode cellular phones drain very low current and engineers developing these apparatuses are constantly trying to further reduce current drains.
When considering the particular mobile telephony sector of embarked electronics, it can be noted that the size and weight of the batteries fitted in these apparatuses are constantly decreasing in spite of the large currents—several hundreds milliamperes—that they have to supply at full load. The search for large outputs—in general greater than 90%—then leads to choosing a switched mode power supply circuit. Indeed, considering a given technology—for example a Very Large Scale Integrated (V.LS.I.) technology for semiconductor circuits etched with an accuracy of 0.13 μm for example, it is noted that nominal voltages for operation—0.9 volt or 1.2 Volts—are much lower than the nominal voltage supplied by a Ion-lithium battery, for example. This voltage is not constant since it tends to vary according to the loading state of the battery and the amount of components prompting said battery at a given moment.
Thus, it is necessary to use a regulated power supply device with a high efficiency if possible, so a designer of a power supply system will naturally turn towards using a Switched Mode Power Supply, which is the only system allowing a high efficiency.
While switched mode power supplies operate satisfactorily at full load, they do not operate in a very satisfactory way for very low loads, which is the case for a cellular phone in suspend mode for example.
FIG. 1 is an illustration of a traditional architecture for a switched mode power supply device, for example for a cellular phone. Typically, a power circuit is comprised of a PMOS transistor 101, associated with a recovery diode 104, and this assembly supplies current to a load (not represented) via a quadripole L-C including an inductive resistor or choke 102 and a capacitor 103. The power circuit is controlled by a control circuit based on a comparator 109 and a ramp generator—comprised of a transistor 114 and a capacitor 115 loaded by a current Ibias. An amplifier circuit, comprised of an operational amplifier (op amp) 106 associated with a feedback circuit, comprising a resistor 107 and a capacitor 108 assembled in parallel, amplifies the error between the output voltage to be regulated Vout and a reference voltage Vref. The output of the amplifier is sent to one of two inputs of the comparator, the second input of the comparator receives the ramp voltage. Thus periodically, at the frequency of a clock signal, the comparator generates a cyclic signal, the cyclic ratio of this signal depends on the variation between the voltage Vout and the reference voltage Vref. Thus, the opening cyclic ratio of power transistor 110 is controlled so that the output voltage Vout gets closer to the desired value Vref.
More particularly, on each clock pulse, the MOS transistor 101, supplying the charging current, starts to conduct and the ramp signal generation starts. When the ramp voltage output from the error amplifier reaches the error voltage value, the error amplifier switches and locks the MOS transistor control. Considering that the voltage Vout drops below reference the value Vref, it should be noted that the error signal tends to rise, causing the cyclic ratio to increase—and in particular factor Ton corresponding to the setting to conduction of the power transistor 101.
Thus, the voltage Vref allows to adjust the desired value for voltage Vout, and it is observed that this output voltage is regulated through the opening period control—Ton—of the feeding transistor.
Although this type of switched mode power supply is really useful in mobile telephony, there still is a problem for very low loads. Indeed, when a phone is in suspend mode and when power drain is particularly low, the error voltage Verr is very low in turn. It is necessary to generate particularly small values for the cyclic ratio and in particular for Ton, but the intrinsic switching times of the toggling circuit and especially the delays of the comparator 109 set limitations.
To reduce the Ton value, a faster comparator allowing to reduce intrinsic switching delays is traditionally used, but this leads to an increased power drain, thereby reducing the value of Ton. Another known way of avoiding the loss of efficiency resulting from the use of a faster comparator consists in acting on the relative value of the cyclic ratio Ton/T by reducing the operational frequency of the switched mode power supply so as to enter a pulse skipping operation mode. Reducing the operational frequency from 1 Mhz to 50 kHz, for example, allows to reduce the cyclic ratio twenty times without having to use a faster comparator. But this means approaching the audio frequency range, which disturbs the operation of the RF part that ensures radio frequency transmission of the phone. Thus it is necessary to prevent switched mode power supplies from operating in pulse skipping mode when the phone is in operating mode, i.e., when RF and audio parts of the circuit are enabled. It is noted that even when these elements are active, the current used can be particularly low.
The desire to use a switched mode power supply—which has the advantages mentioned above in terms of efficiency—for example for a cellular phone, in the whole current range including for lower values, creates the following dilemma: either operation in pulse skipping mode degrades the audio performances of the phone or the increase of the switching speed of the comparator causes the feeding circuit to be less efficient.
Such limitations cripple the development of new cellular phone technologies.
Thus it is desirable to be provided with a switched mode power supply circuit that operates satisfactorily for high loads as well as for very low loads while not disturbing the audio operation of the apparatus and in particular cellular phones.